This application claims priority to German application DE 100 47 548.5, filed on Sep. 22, 2000.
1. Field of the Invention
This invention concerns a procedure for the evaluation of the quality and/or the efficiency of a cable in a cable installation by a current measurement of a leakage current as a parasitic cross current.
2. Discussion of Related Art
It is known, that the parasitic cross current flows as a leakage current between a current carrying conductor at the core of a cable and the cable screen or concentric, which is generally an earth-ground potential. This current reduces the efficiency and/or the Quality of the transmission. By the measurement of this leakage current, the quality of the cable can be determined, additionally a corresponding test criteria can be established. The higher the leakage current, the lower is the quality of the cable.
For the evaluation of the leakage current, it was recommended to connect a DC voltage between the core and the screen of the cable and to measure the resulting flow of the current. For this purpose the cable is disconnected from the net and tested with a DC voltage equivalent to the operating voltage level. The leakage current is measured via a connected current meter. The disadvantage of this procedure is the generation of polarisation effects in the dielectric of the cable insulation resulting in a possible pre-damaging of the cable.
Further known methods are the so called VLF (very low frequency) Test methods. For this method, an alternating voltage with a very low frequency, for example 0.1 Hz is applied. The rising/falling slope of this frequency is in the frequency spectrum of 50 Hz. Significant disadvantages of this technology are interferences of compensation losses and leakage current which prevent a stable and continuous measurement of the leakage current.
Further known technologies as resonance testing have the same disadvantage, that the continuous measurement of the leakage current is not possible. This is a result of continuous re-charging procedures that prevent a stable situation in the cable.
The task of the invention is therefore to provide a procedure and a device of the previously described type, which provides a significantly more reliable evaluation of the quality and/or efficiency of a cable or cable section than previously proposed systems.
A procedure for the evaluation of the quality and/or the efficiency of a cable or a cable segment in accordance with the present invention includes a current measurement of a leakage current indicating the parasitic cross current. This measurement is achieved by supplying between two conductors for example the core and a screen or two cores of a cable, a voltage (VVLF) with alternating polarity and rectangular shape. The periodic duration of the voltage (VVLF) is selected in a way which permits the current measurement of the leakage current at a time shortly before a polarity reversal where the voltage change or rise is zero (xcex94V=0). The leakage current close to the polarity reversal is substantially the same as the parasitic cross current. In some embodiments, the current measurement of the leakage current is performed at a selected time period.
A device on which this procedure can be performed includes a VLF Generator for the generation of a VLF Voltage with changing polarity in substantially a rectangular shape. The VLF voltage is coupled to a cable to be tested and an Analyzing and Evaluation unit for the evaluation and visualization of a leakage current in the tested cable shortly before a polarity change of the VLF voltage.
Further details, characteristics and advantages can be deduced from the following description with respect to the following figures.